Problem: A water tank is $3$ feet high, $7$ feet long, and $6$ feet wide. A solid metal box which is $1$ feet high, $1$ feet long, and $3$ feet wide is sitting at the bottom of the tank. The tank is filled with water. What is the volume of the water in the tank?
Solution: The tank has a volume of $3 \times 7 \times 6 = 126$ cubic feet, and the metal box has a volume of $1 \times 1 \times 3 = 3$ cubic feet. Since there is no water in the box, the volume of the water in the tank is the volume of the tank minus the volume of the metal box. The volume of the water in the tank is $126 \text{ft}^3 - 3 \text{ft}^3 = 123\text{ft}^3$